The present invention relates to nucleic acid molecules encoding cell surface receptors on immune cells and the characteristic peptides that comprise these receptors. More specifically, the present invention concerns the use of synthetic and recombinant peptides comprising natural killer (“NK”) cell surface receptors. The synthetic and recombinant peptides are used to generate monoclonal antibodies that bind a specific NK cell surface receptor called CS1. The binding of the monoclonal antibody to the NK cell surface receptor leads to NK cell activation. In a particular embodiments of the present invention, the monoclonal antibodies are utilized in a method that inhibits the growth of tumor cells.
The immune system is comprised of millions of cells including peripheral blood lymphocytes, monocytes and polymorphonuclear leukocytes, numerous soluble chemical mediators (cytokines and growth factors), and several lymphoid organs including thymus, postnatal bone marrow, lymph nodes, liver and spleen. All of these components work together through a complex communication system to fight against microbial invaders such as bacteria, viruses, fungi and parasites, and against newly arising malignant (tumor) cells. Natural killer (“NK”) cells comprise a subset of peripheral blood lymphocytes that mediate a variety of functions that are important in human health and disease. Initial studies of NK cells concentrated on their ability to kill tumor cells, but like other lymphocytes, NK cells are now appreciated as having a broader role in the first line of host defense against invading pathogens, especially in the earliest phases of host immune responses.
NK-cell activity does not require prior exposure to the pathogen or in vivo clonal expansion, in contrast to the amplified response by cells in the specific effector arm of the immune system (B and T cells). Amplification by clonal expansion requires significant time, usually days to weeks. By then a host may succumb to certain pathogenic infections. NK cells are not static participants in host defense. Although NK cells are not effective against all viruses (for example, they have a less significant role in immune defense against influenza and lymphocytic choriomeningitis virus, even though infection can stimulate NK-cell activity), these cells have been shown to accumulate at sites of viral replication where they produce and release several cytokines such as gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and the hematopoietic colony stimulating factors (CSFs) granulocyte-macrophage (GM)-CSF and interleukin-3 (IL-3). It is not yet known why NK cells are effective against certain pathogens and not others, but there is some evidence to suspect that there may be tissue-specific differences in dominant immune responses. Moreover, the mechanisms by which NK cells mediate their antiviral effects are not clearly understood.
Although not wanting to be bound by theory, the cytolytic function of NK cells and cytokine production are regulated by a delicate balance between signaling through activating and inhibitory receptors. Previous attention in the field has focused on MHC-recognizing receptors that are mostly inhibitory. However, one emerging group of activating NK cell receptors encompasses cell surface molecules of the Ig superfamily homologous to CD2. Although members of the CD2 subset of receptors do not recognize MHC molecules, they still play a major role in NK- and T-cell functions. Two members of this CD2 subset, 2B4 (CD244) and SLAM (CD150) receptors are involved in cellular activation such as lymphoproliferation, cytokine production, cytotoxicity, and invasiveness. The cytoplasmic domains of these transmembrane proteins contain novel tyrosine motifs (T-x-Y-x-x-I/V/A), which differ from those observed in other NK- and T-cell receptors. Another member of the CD2 subset, called CD84, also contains a consensus tyrosine motif, but its function remains unknown. The CD2 subfamily members bind homophilically to other members of the CD2 receptor subfamily, which is unlike the NK-cell receptors that bind MHC class I molecules (Davis et al. 1998). For example, CD2 receptor binds the CD58 receptor and both human and rodent CD48 receptors (Kato et al. 1992; van der Merwe et al. 1993, 1994). The CD48 is a high-affinity ligand for 2B4 and is widely expressed in leukocytes with its soluble form elevated in the blood of patients with lymphoid leukemias and arthritis (Smith et al. 1997). Additionally, the SLAM receptor is a self-ligand that activates T- and B-cells.
The tyrosine motifs in the SLAM and 2B4 receptors interact with a src homology 2 (SH2) domain of a SLAM-associated protein (SAP)/SH2D1A that is believed to stimulate a cytotoxic signaling pathway of the receptor (Sayos et al. 1998; Tangye et al. 1999). NK cells derived from SAP deficient individuals are no longer activated through 2B4. SAP is also essential for the signal transduction of other CD2 family receptors that are differentially expressed on cytotoxic lymphocytes. The lack of function of all these receptors in SAP-deficient individuals results in a complex deficit of NK, T and B cell responses that leads to un controlled infections. Mutations in the SH2 domain of SAP/SH2D1A have been identified as the genetic basis for X-linked lymphoproliferative disease (XLP) (Coffey et al. 1998; Nichols et al. 1998; Sayos et al. 1998). The 2B4 receptor transduces inhibitory signals in XLP patients when stimulated by CD48-positive B cells. Although not wanting to be bound by theory, these data suggest that the cause of XLP is complex and probably due to dysregulation of phosphorylation-dependent interactions at multiple sites, including the receptors and cytoplasmic adaptor molecules. In addition to the 2B4 and SLAM receptors, other receptors that contain the unique tyrosine motif might also contribute to the immune dysregulation seen in XLP.
In an attempt to identify novel cell surface receptors potentially involved in controlling cytolytic function we have identified, cloned and characterized a new member of the CD2 subset of receptors. We have named this new cell surface receptor CS1. This CS1 receptor is localized to the long arm of human Chromosome (Chr) 1 between CD48 and Ly-9 (CD229) and contains two of the unique tyrosine motifs. Disregulated signaling via the CS1 receptor may also contribute to the phenotype of XLP.